Check with mum if she is expecting you to provide a snack or will send one with her DD.
How many hours per day are we talking about?
Are you doing the school run anyway and looking after your own child at the same time? In that case I would charge a lot less. Do you have to drive to pick up the child, or is it just a short walk?
Will you supervise homework, just let them watch TV, or provide some entertainment eg stop at the playground to play, access to a garden to play, toys/games to play, provide materials for colouring etc.
How old is the child? Looking after a 5 year old is harder than looking after a 10 year old.
What arrangements if there is an emergency or the child becomes unwell whilst at your house.
Taking all this into account I wouldnt charge more than £5 a session personally. It's not a iob, and you arent insured.
If it is an informal arrangement and you aren't a childminder. An afterschool club from 3.30pm to 6pm is about £9 where i live including supervised activities and a good quality healthy snack and drinks.